The present invention relates to a process for polymerizing vinyl aromatic monomers, such as styrene, to produce polymers having a high degree of syndiotacticity, and in particular to a process for preparing polymers of vinyl aromatic monomers having a high degree of syndiotacticity using an alkylalumoxane-free catalyst. More particularly still, this invention relates to such a process using catalyst compositions obtained by abstraction of a leaving group from inert Group 4 metal compounds with a borane. The resulting polymers may be usefully employed in the preparation of solid objects and articles such as a moldings, films, sheets and foamed objects by molding, casting or the like process.
In U.S. Pat. No. 4,680,353 there is disclosed a process for the preparation of polymers of vinyl aromatic monomers having a stereoregular structure of high syndiotacticity by the use of certain coordination catalysts. Particularly disclosed were the reaction products of a titanium compound and an organoaluminum compound, especially polymethylaluminoxane. The process disclosed in this patent requires the use of large amounts of the polymethylaluminoxane which is very expensive and difficult to make due to its very complex structure. Also, this process requires an expensive polymer purification system to remove remaining catalyst components due to the use of large amounts of the polymethylaluminoxane.
In EP 277,004 there are disclosed certain bis(cyclopentadienyl) metal compounds formed by reacting a bis(cyclopentadienyl) metal complex with salts of Bronsted acids containing a non-coordinating compatible anion. The reference discloses the fact that such complexes are usefully employed as catalysts in the polymerization of olefins.
In J. Am. Ch. Soc. 113, 3623-3625 (1991) there is disclosed a process for preparation of "cation like" zirconocene polymerization complexes by alkyl abstraction using tris(pentafluorophenyl)borane. The complexes were stated to have activity roughly comparable to typical zirconocene/alumoxane complexes.